Temperature tuning is one of the most common approaches for controlling the wavelength of semiconductors lasers. This is a relatively simple technique to implement since the semiconductor laser chips are often mounted on thermoelectric coolers. By varying the drive current to a chip's cooler in response to a thermistor signal, for example, the output wavelength from the semiconductor laser chip can be stabilized or modulated.
The problem with temperature tuning is its range limitations. The lasers can usually only be tuned over a relatively small range of a few nanometers. As a result, temperature tuning finds limited application to telecommunications systems or spectroscopy applications in which only a small spectral band is to be accessed.
When wider tuning ranges are required, external cavity laser (ECL) configurations are more common. Semiconductor ECL systems usually combine a semiconductor gain chip and an external tuning element that provides wavelength selective feedback into the semiconductor chip.
Most commonly the semiconductor gain chip used in these ECLs is an edge emitting semiconductor optical amplifier (SOA). Either reflective SOAs, in which one of the facets is coated for reflectivity, or more conventional SOAs, in which both facets of the SOA are antireflection (AR) coated, can be used. These coatings prevent the chip from lasing on its own facet's feedback, allowing for the creation of the external cavity configuration in which the spectral makeup of the feedback is controlled by the tuning element.
There are a large variety of ECL cavity configurations. In some examples, the tuning element is located in the middle of the cavity between the reflectors that define the laser cavity. In other examples, the tuning element is located at one end of the cavity, functioning as both a laser cavity mirror and tuning element.
U.S. Pat. No. 6,345,059, entitled, “Short Cavity Tunable Laser with Mode Position Compensation,” and U.S. Pat. No. 6,339,603, entitled, “Tunable Laser with Polarization Anisotropic Amplifier for Fabry-Perot Filter Reflection Isolation” illustrate ECLs using a Fabry-Perot filter tuning element that is located in the middle of the laser cavity.
The location of the Fabry-Perot tuning element in the middle of the cavity can render these ECL designs somewhat complex to manufacture. Isolation is required on either side of the Fabry-Perot tunable filter. This isolation ensures that the laser functions in response to the transmission filter function of the Fabry-Perot tunable filter, rather than its broadband or notch filter function reflection. The transmission of a Fabry-Perot tunable filter spectrally appears as an Airy function with a series of spectrally separated Lorentzian passbands, which are separated by the free spectral range of the filter. Generally, the laser gain chip locks onto one of these passbands to thereby define the wavelength of the ECL. The isolation is important because, in reflection, the Fabry-Perot tunable filter appears as a notch filter, providing very broadband reflection except for the relatively narrow band that is transmitted.
On the other hand, Fabry-Perot tuning elements have some advantages. They can be manufactured using robust micro-electro-mechanical system (MEMS) technologies. Moreover, these ECL cavity configurations can provide relatively broad tuning ranges, only limited by the chip gain bandwidth and the effective bandwidth of the isolation elements.
Isolation for intracavity Fabry-Perot tuning elements, at least those using flat-flat mirrors, can be achieved with simpler configurations. An article by P. Zorabedian, et al. in Optics Letters, Vol. 13, No. 10, October 1988, entitled, “Interference-Filter-Tuned, Alignment-Stabilized, Semiconductor External-Cavity Laser,” discloses a semiconductor ECL using an interference or resonant etalon filter. The filter is angled relative to the optical axis of the ECL cavity. As a result, light is reflected at an angle to the laser cavity's optical axis by the interference filter and is therefore not coupled back into the semiconductor gain medium; this angling avoids the need for discrete intracavity isolation elements. In contrast, light that is transmitted through the interference filter, being on resonance, oscillates within the laser cavity and is therefore amplified by the chip gain medium.
A similar system was disclosed in U.S. Pat. No. 4,504,950 to AuYeung. There, a gap was used to form an intra-cavity etalon to create, in combination with a laser diode, an external cavity laser configuration. A micro-electrical and mechanical translator was used to tune this gap and thereby control the wavelength of the output. The gap was angled relative to the laser cavity in order to ensure that the laser diode locked on the resonant feedback transmission from the etalon and not the off-resonance light.
The lasers described in U.S. Pat. No. 4,504,950 and the Zorabedian article use resonant filters that are constructed from two flat mirrors. Such resonant filters support a continuum of plane wave spatial modes. Thus, these filters can be angled tuned as in the Zorabedian article by changing the angle the modal plane wave makes with the optical axis of the resonator. Or, they can be tuned by varying the optical gap, as disclosed in U.S. Pat. No. 4,504,950. In either case, the flat-flat resonator cavities avoid problems stemming from higher order spatial modes from the filters affecting the filter transmission characteristics. The angle isolation strategy, usable with these flat-flat mirror filters, provides a much simpler cavity design and therefore eases the manufacturing challenges over systems as described in U.S. Pat. Nos. 6,345,059 and 6,339,603.
The isolation problems characteristic of these Fabry-Perot based ECLs are not encountered in some other ECL configurations. Another class of ECLs uses grating or holographic tuning elements. Here, the most common ECL configurations are termed Littman-Metcalf and Littrow. In the Littrow design, the emission from a gain element, such as a laser diode or reflective SOA, is directed to a grating. A portion of the light from the grating is reflected to provide narrow band feedback into the SOA through its AR coated facet, whereas another portion of the beam grazes off of the grating to form the output beam. The spatial distribution of the spectrum by the grating presents only narrow band feedback into the semiconductor chip. In the Littman-Metcalf configuration, the output through AR coated facet of a reflective SOA facet is directed to a grating. A portion of the light grazes to form the output beam. Another portion is reflected toward a turning mirror that then retro-reflects the light to the grating and back into the SOA through the AR coated front facet. Tuning in the Littrow configuration is performed by mechanically rotating the grating, which changes its effective pitch. This tuning, however, also changes the position of the output beam. In contrast, in the Littman-Metcalf configuration, the rotating mirror varies the effective grating pitch and tunes the laser. There is little movement in the output beam since the grating is usually not moved, however.
These grating based ECLs are commercially available. Most recently, relatively compact semiconductor laser systems with MEMS grating tuning have been developed and commercialized.
A further class of ECLs is based on coupled cavity configurations. One example is shown in WIPO Publication No. WO 95/13638, in which a resonant filter is formed between a back facet of a laser chip and a MEMS, electrostatically-driven reflecting element. By changing the size of this rear cavity, it is suggested that the output of the laser is tuned. A similar system is disclosed in a paper entitled, “Coupled-Cavity Laser Diode with Micromachined External Mirror,” by Yuji Uenishi from NTT Interdisciplinary Research Laboratories in Tokyo, Japan, in IEEE/LEOS 1996 Summer Topical Meetings: Optical MEMS and Their Applications, Keystone CO, pp. 33-34, 1996.